An electrolytic capacitor is conventionally constructed by applying the electrolytic oxidation treatment to the surface of a metal foil or a sintered metal composed of aluminum or tantalum to form an insulating film and using the insulating film as a dielectric film.
As the recent main technical requirements for an electrolytic capacitor, there are the reduction of the size, lowering of the impedance with the increase of the frequency of a circuit, a high reliability, the reduction of cost, etc. For these requirements, in the field of an aluminum electrolytic capacitor, an investigation for using a solid electrolyte for an electrolytic capacitor in place of a liquid electrolyte conventionally used for an electrolytic capacitor has been made and as such a solid electrolyte, various electrically conductive polymers such as polypyrrole, polythiophene, polyfuran, and polyaniline are proposed.
More in detail, for example, JP-A-63-173313 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describes that an oxidized dielectric film is formed on a film-forming metal, polypyrrole is deposited thereon by the chemically oxidative polymerization of pyrrole to form an electrically conductive layer, the pyrrole is further electrolytically polymerized utilizing the conductive layer, and the electrically conductive polymer comprising the polypyrrole is deposited on the chemically oxidative polymerized polypyrrole as a solid electrolyte. Also, JP-A-1-253226 similarly describes that an electrically conductive layer comprising manganese dioxide is formed on a dielectric film and polypyrrole or polythiophene is deposited thereon by an electrolytic polymerization to form a solid electrolyte.
However, in these methods as described above, it is necessary to deposite polypyrrole, etc., by an electrolytic reaction on a dielectric film or coating, which is not essentially an electric conductor, and there is a problem in this point. That is, in these methods, the chemically oxidative polymerized film layer or the manganese dioxide layer must be formed on the dielectric film as an electrically conductive layer which is used as an electrode for the electrolytic polymerization and by forming the layer, the electrolytic polymerization can be first practiced.
Thus, JP-A-3-35516 proposes a method of preparing a solution of polyaniline soluble in a solvent, coating the solution on a dielectric film to form a polyaniline film, and immersing the polyaniline film in a solution of a protonic acid to apply a doping treatment. According to the method, an electrically conductive polyaniline film can be formed on the dielectric film by a simple means without need of forming an electrode and hence the method is advantageous in the points of the production efficiency and the cost as compared with the above-described methods.
Furthermore, in the case of using the electrically conductive film comprising the soluble polyaniline as described above, there is sometimes a problem in the adhesion between the dielectric film and polyaniline and it is proposed in JP-A-5-3138 that the problem is solved by mixing the soluble polyaniline with an aromatic polyamic acid, a soluble aromatic polyimide, a polyalkylene glycol, a polymer of a vinyl compound, etc., in an amount of from 1 to 25% by weight, preferably from 2 to 15% by weight, and more preferably from 3 to 10% by weight, based on the weight of the polyaniline.
However, the solid electrolytic capacitor obtained by such a method may show an excellent performance in a capacitance efficiency, tan .delta., a high frequency impedance, etc., but a leak current is considerably large and hence there is a problem that the solid electrolytic capacitor does not have sufficient characteristics as an electrolytic capacitor.
In this case, the leak current is a very important factor in the characteristics of an electrolytic capacitor and for obtaining a practically usable electrolytic capacitor, it is necessary to reduce the value of the leak current as less as possible.